Since the demonstration at the end of the 1970's by Hill et al. of the possibility of writing permanent reflection gratings in the core of optical fibers (named “FBGs” for Fiber Bragg Gratings), intensive development has been carried out on this technology driven by the development of high-end optical fiber applications in numerous fields such as telecommunications, sensing and lasers. Conventionally, FBGs are written by side imprinting a UV interference pattern along a germanium-doped silica fiber that is photosensitive when exposed in the 240-260 nm band. This FBG writing technique was well developed in the 1990's to the point of making high performance gratings written in standard silica fibers (i.e. SMF28) commercially available. The typical process to achieve such gratings combines the steps of chemically stripping the UV opaque polymer coating from the optical fiber, hydrogen loading of the fiber to increase its photosensitivity, writing the FBG using low intensity CW (continuous wave) light to avoid UV-induced weakness of the fiber observed in pulsed regime, thermally aging the FBG to outgas the hydrogen and stabilize its index modulation for long-term operation and finally, recoating the bare fiber with polymer. All of these steps must be performed with great care for the process to be reliable, which ultimately limits the productivity of the FBG fabrication process.
Writing through the coating (WTC) of the fiber is an attractive idea since the stripping/recoat process is particularly complex and time-consuming. Attempts to WTC with UV light were made by using special UV transparent coatings (see L. Chao, L. Reekie, and M. Ibsen, “Grating writing through fiber coating at 244 and 248 nm,” Electron. Lett. 35, 924-926 (1999); and R. P. Espindola, R. M. Atkins, N. P. Wang, D. A. Simoff, M. A. Paczkowski, R. S. Windeler, D. L. Brownlow, D. S. Shenk, P. A. Glodis, T. A. Strasser, J. J. DeMarco and P. J. Chandonnet, “Highly reflective fiber Bragg gratings written through a vinyl ether coating,” IEEE Photon. Tech. Lett. 11, 833-835 (1999)) and by using near UV light where standard polymer coatings are partially transparent (D. S. Starodubov, V. Grubsky, and J. Feinberg, “Efficient Bragg grating fabrication in a fiber through its polymer jacket using near-UV light,” Electron. Lett. 33(15), 1331-1333 (1997)). However, it was not possible to induce a significant refractive index modulation using such techniques in standard silica fibers, thereby limiting this approach to specialty silica fibers with an enhanced photosensitivity and/or a special coating.
At the beginning of the 2000's, a new approach to photosensitivity based on a non-resonant process using the multiphoton absorption of ultrashort infrared pulses demonstrated the possibility of writing FBGs in silica fibers without the need for sensitization, with both the phase-mask (PM) (S. J. Mihailov, C. W. Smelser, D. Grobnic, R. B. Walker, P. Lu, H. Ding, and J. Unruh, “Bragg Gratings Written in All-SiO2 and Ge-Doped Core Fibers With 800-nm Femtosecond Radiation and a Phase Mask,” J. Lightwave Technol. 22, 94-100 (2004)) and the point-by-point (PbP) techniques (A. Martinez, M. Dubov, I. Khrushchev, and I. Bennion, “Direct writing of fibre Bragg gratings by femtosecond laser,” Electron. Lett. 40, 1170-1172 (2004)). The PM technique was particularly well developed and proved its usefulness in writing FBGs in different non-silica materials (S. J. Mihailov, D. Grobnic, C. W. Smelser, P. Lu, R. B. Walker, and H. Ding, “Bragg grating inscription in various optical fibers with femtosecond infrared lasers and a phase mask,” Opt. Mater. Express 1(4), 754-765 (2011)), notably those suitable for mid-infrared applications such as fluorides (M. Bernier, D. Faucher, R. Vallée, A. Saliminia, G. Androz, Y. Sheng, and S. L. Chin, “Bragg gratings photoinduced in ZBLAN fibers by femtosecond pulses at 800 nm,” Opt. Lett. 32(5), 454-456 (2007)) and chalcogenides (M. Bernier, M. El-Amraoui, J. F. Couillard, Y. Messaddeq, and R. Vallée, “Writing of Bragg gratings through the polymer jacket of low-loss As2S3 fibers using femtosecond pulses at 800 nm,” Opt. Lett. 37(18), 3900-3902 (2012)). WTC in silica fibers was successfully demonstrated using 800 nm femtosecond pulses and both the PM (S. J. Mihailov, D. Grobnic, C. W. Smelser, “Efficient grating writing through fibre coating with femtosecond IR radiation and phase mask” Electron. Lett. 43 (8), pp. 442-443, (2007)) and PbP techniques (A. Martinez, I. Y. Khrushchev, I. Bennion, “Direct inscription of Bragg gratings in coated fibers by an infrared femtosecond laser,” Opt. Lett. 31 (11), 1603-1605, (2006)).
Since the PbP technique relies on the formation of void-like defects, the resulting FBGs were reported to present poor mechanical resistance with a mean breaking stress of about 15-20% of the pristine fiber. The PM technique was more successful at this task and demonstrated the possibility of fabricating fundamental order FBGs through both the acrylate and polyimide coating of photosensitive fibers with a mean strain at breakage of respectively 75-85% and 50% of the pristine fiber. (see D. Grobnic, S. J. Mihailov, C. W. Smelser, and R. T. Ramos, IEEE Photon. Tech. Lett., 20, 973, (2008); and 19. S. J. Mihailov, D. Grobnic, R. B. Walker, C. W. Smelser, G. Cuglietta, T. Graver, A. Mendez, Opt. Commun. 281, 5344, (2008)). In both cases, optical damage of the coating was observed and correlated with a saturation of the FBG reflectivity growth.
To date, there has only been one demonstration of WTC of FBGs in unloaded SMF28 fibers by using the PM technique. Referring to C. W. Smelser, F. Bilodeau, B. Malo, D. Grobnic, and S. J. Mihailov, in Advanced Photonics & Renewable Energy, OSA Technical Digest (CD) (Optical Society of America, 2010), paper BThD3, a third order FBG with 90% reflectivity was reported by using a special apparatus combining two short focal length lenses on each part of a third order phase-mask. The mechanical strength of the resulting FBGs was not reported but a structure in the polymer coating was observed, which suggests a degradation of the mechanical properties of the fiber. Since broadband fs-pulses highly disperse angularly after their interaction with a short period phase-mask, the reported approach of using a lens after the phase-mask cannot unfortunately be applied to the writing of fundamental order FBGs, which requires the fiber to be in close proximity to the phase-mask.
In view of the above, there remains a need for a technique of writing high mechanical strength FBGs which alleviates at least some of the drawbacks above.